Danionella is a genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 comprising the danionin

Danionin

The danionins are a group of small minnow-type fish belonging to the family Cyprinidae. Members of this group are mostly in the genera Danio and Devario. They are native to the fresh waters of southeast Asia. Many species are brightly coloured and are available as aquarium fish worldwide...

 species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 D. mirifica, D. translucida, and D. dracula, three of the smallest freshwater fishes.

Distribution

All species of Danionella are found in Myanmar

Myanmar

Burma , officially the Republic of the Union of Myanmar , is a country in Southeast Asia. Burma is bordered by China on the northeast, Laos on the east, Thailand on the southeast, Bangladesh on the west, India on the northwest, the Bay of Bengal to the southwest, and the Andaman Sea on the south....

. D. translucida is described from the Ayeyarwady River

Ayeyarwady River

The Irrawaddy River or Ayeyarwady River is a river that flows from north to south through Burma . It is the country's largest river and most important commercial waterway. Originating from the confluence of the N'mai and Mali rivers, it flows relatively straight North-South before emptying through...

 basin, and D. mirifica was described from the Kaiming area in upper Myanmar.

Description

When first described, Danionella translucida was the smallest ostariophysan

Ostariophysi

Ostariophysi is the second-largest superorder of fish. Members of this superorder are called ostariophysians. This diverse group contains almost 8,000 species, about 28% of known fish species in the world and 68% of freshwater species, and are present on all major continents except Antarctica...

 and the smallest adult vertebrate

Vertebrate

Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

 to inhabit fresh water

Fresh Water

Fresh Water is the debut album by Australian rock and blues singer Alison McCallum, released in 1972. Rare for an Australian artist at the time, it came in a gatefold sleeve...

. Its adult size ranges from 10–12 millimetres (.43–.47 in) SL

Fish measurement

Fish measurement refers to the measuring of the length of individual fish and of various parts of their anatomy. These data are used in many areas of ichthyology, including taxonomy and fisheries biology.-Overall length:...

. D. mirifica gets slightly larger, at about 14 mm (.55 in) SL, but is still one of the smallest freshwater fishes.

Danionella species lack scales

Scale (zoology)

In most biological nomenclature, a scale is a small rigid plate that grows out of an animal's skin to provide protection. In lepidopteran species, scales are plates on the surface of the insect wing, and provide coloration...

, barbels

Barbel (anatomy)

A barbel on a fish is a slender, whiskerlike tactile organ near the mouth. Fish that have barbels include the catfish, the carp, the goatfish, sturgeon, the zebrafish and some species of shark...

, and lateral line

Lateral line

The lateral line is a sense organ in aquatic organisms , used to detect movement and vibration in the surrounding water. Lateral lines are usually visible as faint lines running lengthwise down each side, from the vicinity of the gill covers to the base of the tail...

.

D. mirifica has a single row of melanophores between the pelvic fins and the tips of the cleithra

Cleithrum

The cleithrum is a membrane bone which first appears as part of the skeleton in primitive bony fish, where it runs vertically along the scapula. Its name is derived from Greek κλειθρον = "key ", by analogy with "clavicle" from Latin clavicula = "little key".In modern fishes, the cleithrum is a...

, and there is a lack of melanophores on the underside of the abdomen.

D. dracula reaches 17 mm in length. It is neotonous, lacking 44 bones that develop late in the related zebrafish Danio rerio

Danio rerio

The zebrafish, Danio rerio, is a tropical freshwater fish belonging to the minnow family of order Cypriniformes. It is a popular aquarium fish, frequently sold under the trade name zebra danio, and is an important vertebrate model organism in scientific research.-Taxonomy:The zebrafish are...

. They have teeth made of bone, rather than the true teeth of other fishes, and the males have a pair of boney fangs which the use for sparring for mates. Britz et al. believe the lineage lost true teeth about 50 Ma.

Species

• Danionella dracula Britz, Conway & Rüber, 2009
• Danionella mirifica Britz, 2003
• Danionella priapus Britz, 2009
• Danionella translucida
  Danionella translucida
  Danionella translucida is an extremely small species of cyprinid fish endemic to Myanmar. When described, it was considered to be the smallest freshwater vertebrate. It was collected from the roots of floating aquatic plants in a slow-flowing, shallow stream in the Pegu Division of Myanmar...
  
   T. R. Roberts, 1986